WindowThumbnail: postpone texture deletion to avoid crash

The culprit was the texture that was delete as soon as the item was
delete while they may be used a bit longer by the render thread.

See also: https://github.com/sailfishos/sailfish-office/pull/85

BUG: 464186
BUG: 446874
FIXED-IN: 5.104


(cherry picked from commit 0ad9576f)